Field
The present disclosure relates to the field of tire treads and, more specifically, treads of tires for vehicles of the heavy goods vehicle type, these treads having a tread surface the geometry of which changes with wear.
Description of Related Art
The tread of a tire corresponds to that part of the tire that is situated radially on the outside and extends in the circumferential direction to provide contact between the tire and the roadway and transmit forces. Contact between the tire tread and the roadway forms a contact patch. The purpose of the tread is to provide the tire with sufficient grip to prevent the tire from slipping, it being possible for such slippage to occur under acceleration, braking or cornering. A tire tread comprises voids in the form of grooves delimiting raised elements, such as ribs or blocks. Furthermore, these raised elements may be provided with sipes, namely kinds of cuts delimited by walls of material, these walls being distant from one another by a small enough distance for said walls to come at least into partial contact with one another when they enter the contact patch. These sipes have the purpose of creating additional edge corners of benefit notably in cutting into a layer of liquid that may be present on the roadway, while at the same time maintaining a high quantity of rubbery material. These sipes may, on the walls that delimit them, comprise means for limiting the relative movements of said walls. The collection of grooves and sipes constitutes a collection of edge corners on the tread surface with an initial geometry, it being possible for this initial geometry to remain unchanged whatever the degree of tire wear or to vary with wear in order to optimize the operation of the tread as a function of the remaining thickness of material.
One problem with which tire manufacturers are faced is that of maintaining tire performance throughout the use of the tire, namely irrespective of the level of tread wear. To such an end it has been proposed for the geometry of the edge corners of a tread to be made to alter with wear: document WO 2010072523-A1 describes a tread exhibiting such an evolution with wear.
In that patent application there is described a tire tread which, after partial wear, partially renews the volume of the grooves without thereby adversely affecting the performance of the tire. In order to achieve that, the tread comprises at least one channel formed under the tread surface in the new state, each channel being intended to appear after the tire is partially worn in order to form a new groove useful in draining away water in wet weather.
Thanks to this type of tread design, the depths of the grooves in the new state can be reduced by making new grooves appear after the tread has been partially worn. This then appreciably limits the compressibility of the tread and therefore reduces the dissipation of energy in the tread in the new state, which is beneficial in terms of fuel consumption.
The same patent document WO 2010072523-A1 provides the possible presence of a sipe of the same orientation as that of the channel, this sipe being situated between the channel tread surface. This sipe makes molding and demolding easier.
That same document also provides the presence of a plurality of sipes intersecting the sipe surmounting a channel, namely that these sipes have a direction distinct from the direction of the sipe surmounting the channel.
In order to improve the mechanical integrity of the molding elements placed in the mold for molding these sipes, it is known practice to provide for the presence of reinforcing parts, each reinforcing part molding, in the tread, a cavity opening both onto the tread surface in the new state and into a channel. While these cavities do assist in removing heat from the tread during running by generating ventilation in the channels, foreign bodies such as stones for example may enter them. Once these foreign bodies have entered a cavity, it has been found that they are able to migrate towards a channel and remain there until a sufficient level of wear sees the formation of a new groove. In the meantime, these foreign bodies will be able to attack the wall delimiting the channel and possibly attack that part of the tire that is situated radially underneath the tread, namely the crown reinforcement.
Document WO 2010/030276 A1 discloses such an embodiment. In this example, FIG. 4 in particular shows an intersection of two surfaces, one of these surfaces delimiting a cylindrical well with no variation in geometry in the depth.
Definitions:
The voids ratio of a tread pattern is equal to the ratio between the surface area of the voids (grooves) delimited by the blocks and the total surface area (contact surface area of the blocks and surface area of the voids). A low voids ratio indicates a large contact surface area of the blocks and a small surface area of voids between the blocks.
The voids surface ratio of a tread pattern is equal to the ratio between the surface area of the voids (essentially formed by grooves) delimited by the raised elements (blocks, ribs) and the total surface area (contact surface area of the raised elements and surface area of the voids). A low voids ratio indicates a large contact surface area of the tread and a small surface are of voids between the raised elements.
The voids volume ratio of a tread pattern of a tread in the new state is equal to the ratio between the volume of the voids (formed notably by grooves and cavities) delimited by the raised elements (blocks, ribs) and the total volume of the tread comprising the volume of material to be worn away and the volume of the voids. A low voids volume ratio indicates a low volume of voids relative to the volume of the tread. For each level of wear, a voids volume may also be defined.
The equatorial median plane is a plane perpendicular to the axis of rotation and passing through those points on the tire that are radially furthest from said axis.
A block is a raised element formed on the tread which element is delimited by voids or grooves and comprises lateral walls and a contact face intended to come into contact with the roadway. This contact face has a geometric center defined as the barycenter or center of gravity of the face.
A rib is a raised element formed on a tread, this element extending in the circumferential direction and making a full tour of the tire. A rib comprises two lateral walls and a contact face, the latter being intended to come into contact with the roadway during running.
Radial direction in this document means a direction which is perpendicular to the axis of rotation of the tire (this direction corresponds to the direction of the thickness of the tread).
A transversal or axial direction means a direction parallel to the axis of rotation of the tire.
A circumferential direction means a direction tangential to any circle centered on the axis of rotation. This direction is perpendicular both to the axial direction and to a radial direction.
Axially towards the outside means a direction which is directed towards the outside of the internal cavity of the tire.
The total thickness E of a tread is measured, on the equatorial plane of the tire provided with this tread, between the tread surface and the radially outermost part of the crown reinforcement in the new state.
A tread has a maximum thickness PMU of material to be worn away in running, this maximum thickness PMU being less than the total thickness E of the tread.
The usual conditions under which the tire is run or the conditions of use, are those defined by the ETRTO standard; these conditions of use specify the reference inflation pressure corresponding to the load bearing capability of the tire as indicated by its load index and speed rating. These conditions of use may also be referred to as “nominal conditions” or “normal conditions”.
The contact patch in which the tire is in contact with the roadway is determined with the tire static; from this contact patch a mean value is calculated for the length of the contact patch in the circumferential direction.
A cut generically denotes either a groove or a sipe and corresponds to the space delimited by walls of material that face one another and are distant from one another by a non-zero distance (referred to as the “width of the cut”). What differentiates a sipe from a groove is precisely this distance; in the case of a sipe, this distance is suited to allowing the opposing walls that delimit said sipe to come at least partially into contact at least as the sipe enters the contact patch in which the tire is in contact with the roadway. In the case of a groove, the walls of this groove cannot come into contact with one another under usual running conditions.
The main dimensions of the cross section of a cavity or of a well here mean its diameter if this cavity has a cross section of circular shape, lengths of its minor and major axes if its cross section is oval or its width and length if it is of rectangular cross section. The main direction of a cavity or of a well here means the mean rectilinear direction of the cavity or of the well between the tread surface in the new state, onto which the cavity or the well opens at one of its ends, and a channel formed inside the tread and into which the cavity or the well opens at its other end.